feat: Add PR title linting

[furtib]

Why:
We want to auto-tag every merged PR (maybe even auto-release). For this, we need PR titles (and maybe contents) to follow certain rules.

What:

• Added a CI job to check if the PR title follows guidelines.

Addresses:
#47

[Szelethus]

I understand this is tricky to test, so I'm inclined to commit and see how it works. The only concern I have is related to breaking changes/major releases.

According to https://github.com/amannn/action-semantic-pull-request,

Note that since pull request titles only have a single line, you have to use ! to indicate breaking changes.

and I wonder whether https://github.com/mathieudutour/github-tag-action will pick up on that. Also, I just fear for the transparency of the whole thing. It'd be great if we could forecast whether the PR will be regarded as a patch/minor/major release (for instance, through automatic labeling).

Another option would be to test on a dummy repo.

[furtib]

I'm testing it in a separate repo. I got this for a test PR:

Error: No release type found in pull request title "Test pr commit". Add a prefix to indicate what kind of release this pull request corresponds to. For reference, see https://www.conventionalcommits.org/

Available types:
 - feat: A new feature
 - fix: A bug fix
 - docs: Documentation only changes
 - style: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
 - refactor: A code change that neither fixes a bug nor adds a feature
 - perf: A code change that improves performance
 - test: Adding missing tests or correcting existing tests
 - build: Changes that affect the build system or external dependencies (example scopes: gulp, broccoli, npm)
 - ci: Changes to our CI configuration files and scripts (example scopes: Travis, Circle, BrowserStack, SauceLabs)
 - chore: Other changes that don't modify src or test files
 - revert: Reverts a previous commit

These are case-sensitive!

The auto version increment picks up the feat mark only if merged with squash (we do this).
i tried !feat: ... for a breaking change PR, the linting worked but the version bump couldn't handle this case.

[Szelethus]

i tried !feat: ... for a breaking change PR, the linting worked but the version bump couldn't handle this case.

While that is unfortunate, I think that is a tagging/versioning side fix anyways. I'm in favour of landing this now and ironing out kinks as they come.